Field of the Invention
The invention is directed to a driving technique of a light emitting diode (LED) and more particularly, to an LED backlight module and driving apparatus.
Description of Related Art
In recent years, with the rapid development of semiconductor technology, portable electronic products and flat panel display products also become popular. Among a variety of types of flat panel displays, a liquid crystal display (LCD) has become a main stream in various display products due to its advantages of low voltage operation, no irradiation, light weight, small volume, etc. Generally, since an LCD panel is not self-luminous, a backlight module has to be disposed under the LCD panel to provide a light source for the LCD panel.
Conventional backlight modules are approximately classified as two types. One is a backlight module composed of cold cathode fluorescent lamps (CCFLs), and the other is a backlight module composed of light-emitting diodes (LEDs). Since the LED backlight module may enhance a color gamut of the LCD, various display panel manufacturers generally use the LED backlight module to replace the CCFL backlight module.
The LED backlight module has a plurality of LED strings arranged in parallel, and each of the LED strings is composed of a plurality of LEDs connected in series. Basically, all LED strings can be operated in a system voltage generated by a boost unit, so as to maintain a current flowing through each LED string at the same constant current.
On the other hand, in some applications, there might be requirements for adjusting brightness because of being cooperated with ambient light or the displayed frames being different. At this current stage, a most commonly used method includes providing a dimming signal, filtering the dimming signal through external capacitors and resistors to generate an analog voltage signal, and then comparing the analog voltage signal with a feedback voltage signal from the LED string to control the system voltage, so as to achieve the purpose of dimming. However, in this method, a driving apparatus equipped with the LED string needs additional pins for connecting the external capacitors. Besides, a frequency of the dimming signal cannot be too low, or the filtered analog voltage signal will become distorted and cannot accurately adjust the brightness of the image frames.